A dank winter’s day in the Steel City did not hold obvious promise of the transformative power of nature. But not far from where a gaggle of office workers were enjoying a fag break, a friendly conservation volunteer called Paul ushered me down a ladder to an otherwise inaccessible spot on the banks of the river Don. I found myself, like Alice down the rabbit-hole, in a new sort of country, a lush carpet of floating weeds and a swift-moving ribbon of clear water at my feet muffling the sound of traffic and freshening the air.

Paul and his colleague Karon were checking hidden cameras for Otterly Amazing!, a lottery-funded survey of Sheffield’s small otter population. The motion-sensitive cameras have captured these shy creatures hunting successfully in a river so polluted when I was a boy in the 1970s that it would literally catch fire.

That’s not all they’ve recorded. Two urban explorers, wearing waders and headlamps, were once filmed emerging at the dead of night from the black entrance of an underground millrace near where we stand. And while he removed the camera’s memory card, Paul told me this one had recorded the same moorhen dozens of times in one month, with the final footage showing it in the jaws of a mink.

Mink are a problem for the Otterly Amazing! volunteers because their poo looks much like an otter’s spraint, small elongated pellets about 2cm long. Crouching on the riverbank, Karon scooped one up and held it to my nose. I sniffed cautiously: the scent was fresh, even sweet, almost like hay, indicating that this was an otter’s doings. Mink poo contains ammonia and is altogether smellier.

DNA testing of remains in the spraint show what the otters have been eating: small fish – minnows, gudgeons, sticklebacks – but also brown trout, grayling and pike. The signal crayfish, like the mink an invasive species, is a favourite. More surprising, to me any way, were the traces of heron. Nearby, to great delight, we found an otter’s print in the mud, broader and larger than a mink’s, fresh evidence of the wild downtown.